1. Field of the Invention
Embodiments described herein relate generally to magnetic resonance imaging.
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spins of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation. The aforementioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal.
In MRI, in order to obtain spatial positional information, mutually orthogonal gradient magnetic fields are applied. Thus, a gradient magnetic field generation system in an MRI apparatus includes a gradient magnetic field coil which adds spatial positional information to MR signals by applying a gradient magnetic field in an imaging space where an object is set.
This gradient magnetic field coil produces heat by being provided with pulse electric current during imaging. A gradient magnetic field generation system has various limitations in terms of the total upper limit of electric power, the respective upper limits of electric power in each channel and the like, and does not have enough ability to endure the maximum electric current in every channel (X-axis direction, Y-axis direction and Z-axis direction) concurrently.
Then, in Japanese Patent Application Laid-open (KOKAI) Publication No. 2010-75753 (hereinafter referred to as Patent Document 1), change of the order of imaging protocols and resetting of imaging cessation time are performed in order to keep residual heat of a gradient magnetic field coil equal to or less than an abort level.
Additionally, for example, in the case of reconstructing two-dimensional images, three gradient magnetic fields in a slice selection direction, a phase encode direction and a readout direction are used. Generally, a waveform of a gradient magnetic field is pulsed, and called a gradient magnetic field pulse. A waveform and amplitude of a gradient magnetic field pulse are defined as a part of parameters of an imaging sequence stipulated by an imaging method and imaging conditions.
Out of gradient magnetic field pulses, a gradient magnetic field pulse in the readout direction is to apply a magnetic field having gradient defined by amplitude of a gradient magnetic field pulse.
While a gradient magnetic field pulse in the readout direction is applied, i.e., on-span of a pulse, MR signals (echo signals) emitted from an object are sampled. If the amplitude of the gradient magnetic field pulse during the on-span is constant, the gradient of the magnetic field in the readout direction becomes constant, and this ensures a linear relation between the position of the readout direction and the frequency of MR signals.
In a high-speed imaging method, sampling in the readout direction is performed in a short span. For example, in a high-speed imaging method called EPI (Echo Planer Imaging), a scan (acquisition of MR signals) are performed speedily and consecutively by inverting a gradient magnetic field for each nuclear magnetic excitation.
The pulse waveform of the gradient magnetic field in the readout direction in EPI has a shorter pulse width and a shorter pulse cycle length, as compared with other imaging methods. That is, the frequency component of the pulse waveform of the gradient magnetic field in the readout direction in EPI is high, as compared with other imaging methods.
On the other hand, a gradient magnetic field pulse is generated by applying pulsed electric current to a gradient magnetic field coil. A waveform of the pulsed electric current applied to a gradient magnetic field coil is ideally a block pulse, but actually becomes a trapezoidal wave having a rising edge region and a falling edge region. As a result, a pulse waveform of a gradient magnetic field does not become an ideal block pulse, but becomes a trapezoidal wave having a rising edge region and a falling edge region.
Generally, in high-speed imaging methods such as EPI, a pulse width of a gradient magnetic field pulse is short, and a ratio of a rising edge region and a falling edge region in both ends of a pulse to the entire pulse width becomes high. Therefore, it is proposed to sample data in a rising edge region and a falling edge region as well as in sampling data a flat region of a pulse, so as to use the sampled data for image reconstruction.
The method of sampling data in a rising edge region and a falling edge region is called Ramp Sampling. The Ramp Sampling gives a shorter data acquisition time, as compared with other methods of sampling data only in regions whose gradient magnetic field intensity is flat.
However, raw data sampled at regular time intervals in a rising edge region and a falling edge region do not become equally-spaced in a k-space, because these raw data are sampled while a gradient magnetic field is changing. Then, it is preferable to rearrange the sampled data before reconstruction, in such a manner that the sampled data become equally-spaced in the k-space. This rearrangement processing is generally called regridding.
In the regridding processing mentioned in Japanese Patent Application Laid-open (KOKAI) Publication No. 2010-172383 (hereinafter, referred to as Patent Document 2), a waveform of gradient magnetic field pulse is assumed not a simple trapezoidal waveform but a non-linear waveform, and a non-linear waveform of a gradient magnetic field pulse is calculated based on a waveform of gradient magnetic field current. In the method of Patent Document 2, regridding processing is performed based on this waveform of a gradient magnetic field pulse.
However, in conventional technology, it is difficult to precisely estimate application limits of a gradient magnetic field generation system in terms of electric power depending on an imaging sequence.
Thus, a gradient magnetic field generation system is safely driven under control of keeping a sufficient margin between actual supplied amount of electric current and the application limit value. That is, the supplied amount of electric current to a gradient magnetic field generation system is controlled so as to surely fall below its application limit value.
In other words, in the aforementioned conventional technology, though there is an enough margin from its application limit, a gradient magnetic field generation system is sometimes driven more safely than its application limit. If there was an enough margin up to the application limit of a gradient magnetic field generation system, imaging could be performed under more optimized conditions by increasing a slice number by the value corresponding to the margin, for example.
Thus, it is preferable to accurately judge, in terms of electric load on a gradient magnetic field generation system in MRI, whether an imaging sequence is practicable or not, before performance of the imaging sequence. This is so that imaging is performed under more optimized conditions.
That is, technology to accurately judge whether an imaging sequence is practicable or not in terms of electric load on a gradient magnetic field generation system in MRI has been desired.
The conventional technology mentioned in Patent Document 2 is based on the assumption that a waveform of gradient magnetic field pulse is similar to a waveform of electric current supplied to a gradient magnetic field coil (hereinafter referred to as gradient magnetic field current). That is, if a waveform of gradient magnetic field current is non-linear, a waveform of a gradient magnetic field pulse is assumed to be similar to the non-linear waveform of the gradient magnetic field current.
Then, gradient magnetic field current is actually measured with an ammeter, and regridding processing is performed based on a gradient magnetic field pulse whose waveform is similar (homothetic) to the measured electric current waveform.
Additionally, technology to calculate an output electric current waveform based on an input signal (control signal) to a gradient magnetic field power supply by simulation, and perform regridding processing based on a gradient magnetic field pulse whose waveform is similar to the calculated output electric current waveform is also disclosed.
However, a waveform of gradient magnetic field current and a waveform of a gradient magnetic field actually generated by this gradient magnetic field current do not necessarily accord with each other. Especially, in a waveform of high frequency components like a gradient magnetic field used in high-speed imaging such as EPI, the following fact has been clarified. That is, a difference between a waveform of gradient magnetic field current and a waveform of a gradient magnetic field becomes large, and discordance of a gradient magnetic field waveform in a rising edge and a falling edge becomes conspicuous.
Therefore, MRI technology to accurately calculate actual gradient magnetic field waveforms and perform regridding processing or parameter correction processing with a high degree of accuracy based on the calculated gradient magnetic field waveform has been desired.